SANAA, Yemen – A massive demonstration against Yemen's government turned into a killing field Friday as snipers methodically fired down on protesters from rooftops and police made a wall of fire with tires and gasoline, blocking a key escape route.

At least 46 people died, including some children, in an attack that marked a new level of brutality in President Ali Abdullah Saleh's crackdown on dissent. Medical officials and witnesses said hundreds were wounded.

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The protest in the capital, Sanaa, drew tens of thousands, the largest crowd yet in Yemen's uprising. It began peacefully. A military helicopter flew low over the square just as protesters were arriving after the main Muslim prayer services of the week.

A short while later, gunfire rang out from rooftops and houses, sending the crowd into a panic. Dozens were hit and crumpled to the ground. One man ran for help cradling a young boy shot in the head.

Many of the victims were shot in the head and neck, their bodies left sprawled on the ground or carried off by other protesters desperately pressing scarves to wounds to try to stop the bleeding.

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The United States, which supports Yemen's government with $250 million in military aid this year alone to battle one of al-Qaida's most active franchises, condemned the attack on protesters.

"Those responsible for today's violence must be held accountable," President Barack Obama said. He called on Saleh to adhere to his public pledge to allow peaceful demonstrations.

Instead, Saleh declared a 30-day nationwide state of emergency that formally gave his security forces a freer hand to confront demonstrators. The declaration bars citizens from carrying and using weapons.